Electron tubes such as traveling-wave tubes (TWT's) conventionally include an evacuated housing in which a beam of electrons is focused and projected axially from a cathode to a collector through a section of a waveguide. Amplification results from interaction between the electron beam and the axial component of electrical field of a wave propagated through the internally provided circuit.
A conventional technique for focusing the electron beam employs periodic-permanent-magnets external to the vacuum envelope of the tube with magnetic shims utilized between the magnets to direct the magnetic focusing flux toward the electron beam and to decrease leakage flux. However, relatively large magnets and shims are required to produce the required magnetic field in the center of the traveling-wave tube and relatively complex and expensive mounting arrangements are required to support the magnets and shims externally in a predetermined fashion about the tube envelope.
Further increasing the complexity and cost of such tubes of the prior art is the requirement for prefabricating and supporting the circuit within the tube envelope.
In our co-pending application Ser. No. 497,787, filed 15 Aug. 1974, for "A Low-Cost Periodic Permanent Magnet and Electrostatic Focusing Scheme for Electron Tubes," we teach a form of traveling-wave tube wherein the periodic-permanent magnets are positioned within the evacuated housing to reduce the size and the cost of the magnetic and electrostatic focusing and control systems for such tubes.
In this co-pending application, a ceramic substrate is shown that includes separate slots for each of the magnets as well as separate slots for the electrical control circuitry. This aspect of the invention provides that the focusing magnets are located inside the vacuum tube envelope and, therefore, closer to the beam. In this co-pending invention, beam focusing is achieved without the necessity of magnetic shims and mounting brackets and the overall supporting arrangement for the permanent magnets is materially simplified. However, the interspacing of the magnets and the control circuit element requires more space between the electrical elements and a more complex ceramic substrate than that of the subject invention.